The present invention relates generally to printing, and more particularly to a method for determining the ink drop velocity of a carrier-mounted printhead.
Known printers include a printer, such as an inkjet printer, having a carrier-mounted printhead with print nozzles used to print ink on a print medium. The printhead carrier moves the printhead back and forth along a scanning axis at a predetermined gap above the print medium. Printing may be from left to right, from right to left or bidirectional (i.e., from left to right and from right to left). The print medium is advanced in a direction perpendicular to the scanning axis when the printhead has finished printing a scan line in one or more print passes.
The printhead is fired with enough energy to eject ink from the print nozzles at an ink drop velocity (defined to be the ink drop velocity relative to the printhead) having a direction along the ink ejection direction from the printhead to the print medium and having a magnitude typically in the range of 250 to 700 ips (inches per second) with 400 ips being an average number for a typical inkjet printer. From printhead lot to printhead lot, there are substantial variations in the amount of energy needed to attain this magnitude of the ink drop velocity. During printing, the ink drop velocity is assumed to have a particular magnitude. This assumed magnitude of the ink drop velocity is used to determine where the ink drop will land on the print medium if fired from a printhead having a predetermined gap and a known printhead carrier velocity.
This assumed magnitude of the ink drop velocity is often wrong. The effect is that ink drops do not land exactly where intended. It does not matter if the actual magnitude of the velocity is greater or less than the assumed magnitude; the net effect is still the same. Known printhead alignment procedures can compensate for some of this variation, but if the actual magnitude of the ink drop velocity could be determined, print quality could be enhanced. Known techniques for determining the ink drop velocity include measuring the time it takes for the ink drop to pass between two optical drop sensors spaced a predetermined distance apart above the print medium.
What is needed is an improved method for determining the ink drop velocity of a carrier-mounted printhead.
A first method of the invention is for determining the ink drop velocity of a carrier-mounted printhead located at a predetermined gap above a print medium and includes steps a) through d). Step a) includes printing a first ink mark on the print medium using the printhead with the printhead carrier moving along a scanning axis at a first printhead velocity. The printhead begins ejecting ink corresponding to the first ink mark at an ink-ejection position along the scanning axis. Step b) includes printing a second ink mark on the print medium using the printhead with the printhead carrier moving along the scanning axis at a second printhead velocity which is different from the first printhead velocity. The printhead begins ejecting ink corresponding to the second ink mark at the ink-ejection position plus a predetermined offset distance, and the second ink mark is spaced apart from the first ink mark. Step c) includes measuring the distance between the first and second ink marks. Step d) includes calculating the ink drop velocity using the measured distance, the predetermined offset distance, the first and second printhead velocities, and the predetermined gap.
A second method of the invention is for determining the ink drop velocity of a carrier-mounted printhead located at a predetermined gap above a print medium and includes steps a) through d). Step a) includes printing a first pattern of first ink marks on the print medium using the printhead with the printhead carrier moving along a scanning axis at a first printhead velocity. The printhead begins ejecting ink corresponding to the first ink marks at equally-spaced-apart ink-ejection positions along the scanning axis. Step b) includes printing a second pattern of second ink marks on the print medium using the printhead with the printhead carrier moving along the scanning axis at a second printhead velocity which is different from the first printhead velocity. The printhead begins ejecting ink corresponding to the second ink marks at the ink-ejection positions plus a predetermined offset distance, and the second ink marks are spaced apart from, and interleaved with, the first ink marks. Step c) includes measuring the distance between adjacent and ink-ejection-position-corresponding first and second ink marks using an optical reflective sensor mounted on the printhead carrier. Step d) includes calculating the ink drop velocity using the measured distance, the predetermined offset distance, the first and second printhead velocities, and the predetermined gap.
A third method of the invention is for determining the ink drop velocity of a carrier-mounted printhead located at a predetermined gap above a print medium and includes steps a) through c). Step a) includes printing an ink mark on the print medium using the printhead with the printhead carrier moving along a scanning axis at a printhead velocity. The printhead begins ejecting ink corresponding to the ink mark at an ink-ejection position along the scanning axis. Step b) includes measuring a distance between the ink-ejection position and the ink mark. Step c) includes calculating the ink drop velocity using the measured distance, the printhead velocity, and the predetermined gap.
Several benefits and advantages are derived from one or more of the methods of the invention. Measuring ink drop velocity will insure high quality printing with a more accurate placement of the ink drops on the print medium. In examples of the methods which use a printer""s existing printhead-carrier-mounted auto-alignment optical reflective sensor in the distance measuring step, ink drop velocity can be measured without requiring additional printer hardware.